Rainbow Cake with Lemony Swiss Buttercream

I celebrated my birthday yesterday, and let me tell you it’s been fabulous. I can’t stop smiling from all the wonderful phone calls, countless FB messages, and the endless celebrating for my birthday. I also share it with my wonderful cousin, a handful of friends and a current coworker! I know at least eight people who share my lucky birthday, but what a great day it is right? My husband sent beautiful tulips and stargazers to my work, and enclosed it with the sweetest heartfelt message. My coworkers even had champagne and cake for my birthday. My boss took me out for sushi for lunch and I still have plans this weekend to celebrate my birthday with more of my friends. Luckily, I also had my birthday night off from cooking dinner as well, and my husband prepared a delicious Ina Garten inspired steak dinner. He’s fancy like that.

Birthdays and cakes go hand in hand and I wanted a spectacular and festive birthday cake. Birthday cakes are like rainbows and puppy dogs, it should be wonderful and everything sweet. Something that I would truly enjoy making as well because I had pondered the idea of buying something as well, but I wanted it to be about me. What did I want to eat on my birthday? I loved the beautiful and playfulness of this rainbow cake, it screamed birthday cake and a six layer cake was something that I hadn’t tackled yet either.  As well as the buttercream frosting, I haven’t mastered that as well. I was ready for the challenge and absolutely tickled pink to bake this beauty up.

I might have a had a little baking meltdown when baking and preparing this cake. I accidently put in too much pink food coloring and it looks more orange. I know there isn’t pink in a rainbow, but it was my rainbow right? I didn’t read the frosting recipe all the way, and forgot about 2 sticks of additional butter. I read 2 cups wrong and saw it as 2 sticks of butter. I needed a total of 4 sticks of butter for the frosting, and luckily I only had 2 more sticks of unsalted butter left in the fridge. It was my d’oh moment. Lessons learned; read the entire recipe before and after you bake and add food coloring at small doses at a time. I neglected to read the entirety of the frosting recipe but in the end everything turned out for one magnificent and colorful cake, and I already have requests from friends to make it for their birthday. It was everything that I had wanted in a birthday cake and more.

Birthdays are nature’s reminders that we should eat more cake, and this was truly a beautiful and scrumptious cake to be eaten and savored. The rainbow cake is a little dense with all six layers but the hint of lemon buttercream is like velvet in your mouth. Smooth with each bite. Enjoy!

Rainbow Cake with Lemony Swiss Buttercream
Adapted from Martha Stewart
Printable Recipe
Vegetable shortening3 cups all-purpose flour4 teaspoons baking powder½ teaspoon salt1 cup unsalted butter, (2 sticks) room temperature2 ⅓ cups sugar5 large egg whites, room temperature2 teaspoons vanilla extract1 ½ cups milk, room temperatureFood Gel Coloring: Red, orange, yellow, green, blue, purple (I used pink…but I added too much)

Lemony Swiss Meringue Buttercream10 large egg whites2 cups sugar2 cups unsalted butter, (4 sticks) room temperature2 teaspoons lemon extract


Preheat oven to 350 degrees. Coat the six 8-inch round cakes pans with shortening (or whatever amount of cake pans that you have. I only have three, reuse what you have). Line each of the cake pans with a fitted parchment paper, coat again with shortening.

In a large bowl, whisk together the flour, baking powder, and salt. Set aside.

In the bowl of the stand mixer, cream together the butter and sugar. Slowly add the egg whites and vanilla extract and mix until fully combined. Alternate adding in the dry ingredients and milk ingredients with the butter mixture. Mix until fully combined.

Make sure you have at least 6 medium size bowls and divide the batter evenly. A food scale would help and I figured about  Add enough food coloring to each bowl until the desired shade is reached. Transfer batter to pan. Bake for 15 minutes, or until a cake taster comes out clean.

Remove cakes from oven and transfer to a cooling rack. Let cool for 10 minutes. Invert cakes on cooling rack and cool completely.

To make the frosting, cook egg whites and sugar in a small saucepan over medium heat in a heatproof bowl, whisking constantly until sugar has completely dissolved. Once the temperature has reached 140 degrees, transfer to the bowl of the stand mixer fitted with the whisk attachment. Mix egg white mixture on high speed until it reaches room temperature. With the mixer on low, slowly add butter, one tablespoon at a time. Mix until fully combined. Remove whisk attachment, and switch back to paddle attachment. Increase the speed to medium-high and beat until buttercream comes together, add the lemon extract.

Using a serrated knife, even out the cake and trim off any excess. Place a large piece of wax paper on the serving cake plate or on top of a lazy susan. Place the purple layer on the cake plate. Spread a dollop (½ cup or more) of buttercream filling and spread it evenly with a cake spatula. Repeat process with blue, green, yellow, and orange layers.

Place the remaining red layer on top, bottom-side up. (Which I neglected to do but you can barely tell). Brush away any excess crumbs with a pastry brush or your hands. Using an offset spatula, cover the top and sides with a thin layer of frosting. Refrigerate until set, abut 30 minutes.

Using the offset spatula, cover cake again with remaining frosting. Garnish with colored sprinkles if desired.

Review: Best birthday cake ever. How am I going to top that for next year? Better question, what cake shall I make for my husband’s birthday next month?

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